Mode selection by synchronous pumping of a wagon wheel optical cavity

ABSTRACT

A laser cavity structure is disclosed which pertains to laser resonator geometries possessing circular symmetry, such as in the case of disk or spherical lasers. The disclosed invention utilizes a very-high finesse Bragg reflector (VHF-BR) thin film reflectors of many layer pairs of very small refractive index difference, the VHF-BR deposited on a surface of revolution, thereby forming an optical cavity. These dielectric reflectors are disposed in such a way as to allow selection of preferred low order modes and suppression of parasitic modes while allowing a high cavity Q factor for preferred modes. The invention disclosed, in its preferred embodiments, is seen as particularly useful in applications requiring high efficiency in the production and coupling of coherent radiation. This is accomplished in a cavity design that is relatively compact and economical.

RELATED APPLICATIONS

Divisional of U.S. application Ser. No. 10/968,280, filed Oct. 18, 2004now U.S. Pat. No. 7,408,969 continuation-in-part of U.S. applicationSer. No. 09/839,254, filed Apr. 20, 2001 now U.S. Pat. No. 6,807,216which claims benefit of U.S. Prov. Appl. No. 60/236,446, filed Sep. 29,2000

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to the field of modediscrimination means in laser cavities, and in particular, modediscrimination in macroscopic cavities wherein a vast number of modesmay otherwise be sustained.

2. Description of the Related Art

The present invention relates generally to the field of lasers andoptical resonator design, and in particular, to the fields of disk andspherical lasers. Also, the invention relates to cavity structuredesigns that utilize multi-layer dielectric (MLD) thin film reflectorsthat provide a high degree of mode selection. Inventive matter disclosedherein is related to co-pending U.S. patent application Ser. No.09/839,254, filed Apr. 20, 2001.

Laser cavities of the disk and spherical geometries have become anincreasingly intensive field of research; in particular, for such lasersthat are fabricated on a miniature or microscopic scale. In the lattercase, the predominant means of cavity reflection is through totalinternal reflection (TIR), which provides an extremely high cavity Q.Such reflective means normally manifest in “whispering modes,” whichpropagate at angles below the critical angle for TIR. These microdiskand microsphere lasers are very effective in cases involving evanescentcoupling to an adjacent dielectric structure; however, they are known tocontain a very large number of competing high-order modes. In addition,the coupling of these whispering modes for useful work is difficult forapplications not utilizing evanescent coupling.

In recent years, theoretical studies have been performed on thedevelopment of derivation methods for cylindrical and sphericalmultilayer structures, which are aimed at providing an accuratedescription of the reflection coefficients and modal characteristics ofthese cavities. These studies address circular confinement structureswith cavity dimensions on the order of the wavelengths studied. However,none of these studies are found to address the issues of applyingsimilar circular Bragg reflectors for larger cavities of the scale usedfor gas and larger solid state cavities. Furthermore, these previousstudies also entertain only the use of conventional MLD filters, with alarge real refractive index difference, n_(H)−n_(L)=Δn>1, for the layerpairs, and with an accordingly small number of layers required for highreflection.

The use of interference structures to enable high spectral resolvingpower in reflecting coatings has been described by Emmett (U.S. Pat. No.4,925,259), wherein a very large number of alternating dielectric layerspossessing a very small difference in refractive indices is used forapplication in high power flashlamps. The described coatings areutilized primarily for providing a high damage threshold to the highirradiance experienced in the flashlamp enclosure, as well as forobtaining a well-resolved pump wavelength for use in the describedflashlamp.

The control of transverse modes in semiconductor lasers, primarilyVCSEL's, has been reported by several research groups in the lastdecade. These latter reports utilize a circular Bragg grating structureas a complement to the planar Bragg mirrors of a conventional, high Qsemiconductor cavity. Such circular Bragg gratings do not form theinitial resonant cavity, but rather, aid in controlling relatively lowQ, transverse modes of an existing Fabry-Perot structure. In such cases,the resultant control of transverse propagation may allow loweredthresholds, or enhanced stability.

Earlier, large-scale, laser designs of a circular geometry operated onvery different principles than the microlasers, utilizing primarily gaslaser mediums and metallic reflectors. In these earlier designs, opticalpower could be coupled for useful work at the center of the cavity, suchas for isotope separation, or by using a conical reflector. Since, inthese latter cases, laser modes that concentrated energy at the cavity'scenter were needed, some means for blocking the whispering-type modeswas generally required. Such mode suppression was usually accomplishedthrough radial stops; however, these stops only provided the mostrudimentary mode control, in addition to hampering the efficientoperation of the laser. Because of such issues, disk and sphericallasers have not supplanted standard linear lasers for any applicationsrequiring substantial optical power or a high degree of mode selection.

SUMMARY OF THE INVENTION

p number of high-index and low-index layer pairs in VHF-BR

n_(g) refractive index of gain volume

n_(h) refractive index of high-index layer in VHF-BR

n_(l) refractive index of low-index layer in VHF-BR

Δn difference in refractive index between n_(h) and n_(l)

n_(l)/n_(h) ratio in refractive index of n_(l) to n_(h)

r radius of optical cavity (outside edge of the VHF-BR)

d thickness of gain volume

K average optical extinction of the VHF-BR

Δθ_(r) degrees of solid cone of reflection in optical cavity

θ_(o) central angle of highest reflectance in optical cavity

θ_(n) angle-of-incidence of a non-preferred mode in optical cavity

λ_(c) wavelength of mode in optical cavity

λ_(p) wavelength of pumping source

ω_(p) frequency of pumping oscillations

ω_(c) frequency of cavity oscillations

N number of pump sources

θ_(s) radial angle between pump sources as referenced from cavity axis(9)

θ_(p) phase angle between peak emissions from separate pump sources.

DEFINITIONS

The term “surface-of-revolution”, in the present disclosure will havethe same meaning it does in mathematics, as a the three dimensionalsurface created by rotating a given profile around an axis, such asrepresented by cones, spheres, cylinders, etc. A “surface ofrevolution”, in the present disclosure, will accordingly refer tosurfaces of physical structures that form a surface of revolution.

In the detailed descriptions of the present disclosure, it shall beunderstood that the terms “VHF-based cavity” and “optical cavity” areused to indicate structures specifically pointed out to form suchcavities.

The term “finesse” shall be understood to refer to its usage as ameasure of an angular sensitivity obtained through a very large numberof interfering waves, whether such waves are made colinear bydiffraction or reflection.

Angle-of-incidence shall refer to the angle at which a paraxial ray oflight is incident upon a surface, with orthogonal incidence being 0°.

A novel laser cavity is disclosed for use in such applications as lasersand light amplifiers in general. In its first preferred embodiment, thedisclosed cavity comprises a cavity mirror structure that provides asingle surface of revolution. The cavity volume is defined by thissurface of revolution, and contains the gain medium. Unlike prior artdisk and/or spherical lasers possessing circular cavities, the presentinvention does not rely on total internal reflection (TIR) or metallicreflectors to provide a high cavity Q-factor (and a broad range ofhigh-order modes). The laser cavity design of the present inventionavoids use of these cavity confinement methods. In the optical resonatorof the present invention, interference-based multilayer dielectric (MLD)reflectors are constructed that can possess unusually narrow reflectionpeaks, corresponding to a degree of finesse (finesse designatinginterference-based resolving power) usually associated with MLDtransmission filters of the Fabry-Perot type. The high-finesse MLDreflectors of the present invention conform to the surface of revolutionof the cavity mirror structure, allowing a high degree ofangle-dependence for selective containment of cavity modes. Thesefilters are disposed in such a way as to allow preferred-low order modes(lower order modes being represented in the present disclosure as thosecorresponding to near normal incidence radiation) and suppression ofparasitic modes while allowing a high cavity Q factor for the modesselected.

For a multi-layer dielectric (MLD) coating consisting of alternatinglayers, where all layers have an optical thickness equal to aquarter-wave of light at the wavelength of interest, the reflectance maybe described according to:

$\begin{matrix}{R = \left( \frac{1 - {\left( {n_{H}/n_{L}} \right)^{2p}\left( {n_{H}^{2}/n_{s}} \right)}}{1 + {\left( {n_{H}/n_{L}} \right)^{2p}\left( {n_{H}^{2}/n_{s}} \right)}} \right)^{2}} & (1.1)\end{matrix}$wherein the index of refraction for the substrate is n_(s), the twolayer indices are n_(H) (high index) and n_(L) (low index), and thenumber of pairs of alternating layers is p. As is evidenced by equation(1), a higher reflectance may be achieved through the implementation ofa greater difference in refractive index Δn=|n₂−n₁|. Properties ofquarter-wave MLD's, such as represented by eqn (1.1), are well-exploredin the prior art of thin film filters. More exhaustive explanations maybe found in Angus MacLeod's book, Thin Film Optical Filters, 2^(nd) Ed.,McGraw-Hill, 1989, pgs. 158-187, which is included herein by reference.High reflectance is thus normally achieved by maintaining Δn at arelatively high value. However, as equation (1) suggests, highreflectance may also be achieved by depositing many layer pairspossessing a relatively low difference in their refractive indices. Asthe index difference decreases, many more pairs of alternating layersmust be deposited to maintain reasonable reflectance. At the same time,this latter approach will result in a decrease in the bandwidth of lightreflected by the resultant coating. The present invention utilizes MLDcoatings which obtain high reflectance from an unusually low Δn; this isaccomplished by maintaining a high degree of control over the propertiesof each layer through an unusually high number of iterations, p, of thelayer pair. With well-controlled film characteristics, the reflectanceof the resulting MLD coating may be fabricated to have a quite narrowbandwidth, typically in the order of nanometers.

Unlike the typically high Δn of MLD-based Bragg reflectors used insemiconductor laser cavities of the prior art, the very high number p oflow Δn layer pairs provides for unique means of obtaining mode selectionin cavities of the present invention, due to the high sensitivity ofreflectivity to angle-of-incidence. Accordingly, the MLD-based Braggreflectors of the present invention will be herein termed Very-HighFinesse (VHF) Bragg reflectors, henceforth referred to as VHF-BR's, forpurposes of teaching the novel embodiments disclosed herein. Themulti-layer dielectric VHF-BRs described herein are, of course,physically and operationally distinct from grating structures that havealso been called “Bragg reflectors” and provide an associated highfinesse, but operate by diffraction. The disclosed VHF-BR is alsodistinct from multilayer Bragg reflectors incorporating much larger Δnand smaller p, so that sensitivity to angle-of-incidence is not adequatefor providing useful mode-selection properties in the optical cavitiescontemplated herein. The disclosed VHF-BR-based cavities disclosed willalso be found distinct from the multilayer Bragg reflectors and cavitiesutilized in “Bragg fiber gratings” that utilize high-index cavities forwave-guiding and in which divergence of a propagating beam is determinedby the fiber structure.

A characteristic of the VHF-BR utilized in the present invention is theangle-dependence of the reflection peak. As the VHF-BR is irradiated atincreasingly oblique angles of incidence, the spectrally narrowreflection peak will be shifted toward increasingly shorter wavelengths.While the degree of this latter peak shift will depend on such issues asphase dispersion and the change in optical admittance with increasinglyoblique incidence, the fractional shift in the peak transmittance willchange generally with the phase thickness shift. As such, the fractionalshift in peak transmittance will be slightly less than cos θ, where θ isthe angle from normal incidence. As the angle of incidence, θ,increases, the magnitude of the reflectance peak will generallydecrease, as well. This decrease in magnitude will be made greater bythe formation of the VHF-BR as a circular reflector, so that non-normalincident propagation suffers from a lack of coherence in its reflectionfrom the curving layers of the reflector.

The aforementioned characteristics of these VHF-BR's are utilized in thepreferred embodiments of the present invention. In accordance with theillustrated preferred embodiments, a novel laser cavity structure isdisclosed herein that effectively utilizes the sensitivity of theaforementioned coatings to angle-of-incidence when these same coatingsare irradiated with quasi-monochromatic light. This is normallyaccomplished through the use of a cavity mirror that conforms to asingle surface of revolution. High confinement is achieved through noveluse of the highly angle-dependent VHF-BR's. Thus, instead of utilizingTIR or metal films, which both provide wide acceptance angles to highorder cavity modes, the present invention utilizes external reflectionand narrow acceptance angles to increase the stability of selected,lower order, cavity modes.

Because the present invention does not rely on TIR or metallic films toprovide high confinement for various laser modes, it is designed with afundamentally different set of requirements for the refractive indicesof its individual components. In contrast to the disk and sphericallasers of the prior art, the gain medium—or, equivalently, the volume inwhich it resides—in lasers of the present invention should preferablypossess an effective refractive index, n_(g), lower than that of theimmediately surrounding medium. As such, the high index layers of theVHF-BR of the present invention must have a refractive index, n_(H),greater than that of the gain volume.

In one preferred embodiment, the present invention is particularlysuited to operation with excimer gases as the gain medium, due to themode-selection means providing substantially improved cavity quality forthe preferred modes, while allowing very little cavity confinement forunwanted cavity modes, so that conventional unstable resonators commonto excimer laser design are no longer required to provide useful modediscrimination.

In another embodiment, the invention provides a unique configuration forcoupling laser radiation from the center of a solid state laser cavity.The latter embodiment includes a solid state gain medium that is formedinto an annular disk geometry having diameter larger than its thickness,so that efficient cooling of the medium may be performed through coolingof first and second opposing faces of the medium. The disclosed solidstate gain medium further includes a first surface-of-revolutioncomprising its outer edge, whereon a VHF-BR is formed for cavityconfinement of optical energy. The disclosed solid state gain mediumalso has a second surface-of-revolution comprising the inner edge of theannular disk, which provides a means for out coupling optical energyfrom the annular disk. Specific means for outcoupling optical energyfrom the annular solid state gain medium include a disclosed combinationof a concentric conical reflector and beam condensation means.

In yet another embodiment, the invention provides mode selection meansin solid state gain media that are polygonal in shape; for example,rectangular, pentagonal, trapezoidal, etc. Improved amplification andlasing characteristics are provided in such polygonal gain media throughimplementation of a VHF-BR reflector on one or more planar facets of thegain medium. This embodiment particularly includes a rectangular “slab”geometry that provides for an improved spatial uniformity in absorptionand gain, thereby decreasing mechanical stress and thermal lensingeffects. The disclosed slab geometry is incorporated in bothamplification modules and in a slab laser design. The embodimentprovides improved thermal characteristics over previous slab laserdesigns, and uniform pumping and absorption within the gain material maybe achieved without use of “zig-zag” or other folded-cavity beam paths.

In yet another embodiment of the present invention, low loss and veryhigh finesse are achieved in the VHF-BR through use of interleavedlayers of polymer thin films as the low-index layer, the high-indexlayer, or both. This disclosed polymer-based VHF-BR I preferablycomposed of polymer high-index layers and inorganic (such as silica)low-index layers. In the polymer-based VHF-BR, a very large number oflayers (>1,000) may be deposited without the surface roughening, loss ofspecularity, and optical absorption that is a common problem whendepositing thick all-inorganic VHF-BR's.

In an alternative embodiment, a deformable cavity material is disclosedproviding novel mode-selection properties, the material providing aflexibility allowing it to conform to a variety of cavity shapes. Also,it is seen as particularly advantageous that the flexible cavitymaterial can be fabricated with a tailorable elasticity that allows thereflective properties to be tuned via an applied tensile strain.

In another embodiment, the invention provides a means for incorporatingthe VHF-BR and associated optical cavities into a dielectric layer thatis formed on a planar substrate. In this embodiment, a VHF-BR is formedthrough modification of the dielectric layer to form therein aconcentric pattern of many ring-shaped regions possessing anindex-of-refraction that is slightly higher than that of the originaldielectric layer, thereby forming a VHF-BR structure enclosing anumodified central region of the dielectric layer, so that an opticalcavity is formed by the VHF-BR and the central region. In yet anotherembodiment, the planar substrate is preferably a polished wafer,preferably made of single-crystalline silicon (e.g., Czochralski orBridgeman grown), and the dielectic layer is capped with a siliconlayer, resulting in a novel silicon-on-insulator (SOI) substrate thatincorporates an optical cavity within an insulator layer of the SOIsubstrate. In these embodiments that incorporate a modified dielectriclayer, a gain volume is alternatively formed within the central regionof the disclosed optical cavity by such methods as diffusing a dopantion into the central region of the dielectric layer.

In another embodiment, a multitude of pump sources are positioned overthe gain volume of the optical cavity, in a preferably symmetricpattern, so that the multitude of pumps may be powered with a cyclicalpower signal. The frequency of the cyclical power, as well as the shapeof the pattern, result in a method and structure for preferentiallyproducing gain in a desired mode of oscillation in the optical cavity.

Other objects of the present invention follow.

One objective of the present invention is to provide a laser cavitystructure that allows high thermal stability.

Another objective of the present invention is to provide a disk orspherical laser cavity structure that discourages the establishment ofwhispering modes

Another object of the present invention is to provide a laser cavitystructure which allows mode selection through the use of all-dielectricreflectors of unusually high finesse.

Yet another object of the present invention is to increase the stabilityof conventional laser cavity structures through the suppression ofwalk-off modes.

Another object of the present invention is to provide a laser cavitystructure that allows a low threshold to lasing.

Another object of the present invention is to provide a means forirradiating a photo-absorbing medium from a continuous 360-degreeperiphery.

Another object of the present invention is to provide a laser cavitystructure that allows efficient and reliable mechanical design.

Another object of the invention is to provide a laser cavity structurethat may be readily implemented for large-scale cavities.

Another object of the invention is to provide a laser cavity structurewherein the absorption edge of an incorporated material preventsunwanted laser modes.

Another object of the invention is to provide a laser cavity structurewith an unusually high effective numerical aperture.

Another object of the invention is to provide an excimer-based lasercavity that provides inherently better cavity confinement of preferredmodes relative to unstable resonators of previous excimer lasers.

Another object of the invention is to provide an excimer laser thatutilizes a circular electrode configuration for high operationalstability.

Another object of the invention is to provide an excimer laser thatenables the use of cone elements for extracting energy.

Another object of the invention is to provide an excimer laser cavitywherein an absorption edge of an incorporated material limits unwantedlasing.

Another object of the invention is to provide an excimer laser thatenables irradiation of circularly symmetric articles.

Another object of the invention is to provide an excimer laser thatenables irradiation of dispersed media.

Allows for an excimer laser to be operated with excellent modeselection, without the use of the unstable resonators used in the priorart.

allows for electron discharge pumping to be implemented in a morestable, higher symmetry configuration.

Another object of the invention is to provide an excimer laser thatutilizes a circular electrode configuration for high operationalstability.

Another object of the invention is to provide an excimer laser thatenables the use of cone elements for extracting energy.

Another object of the invention is to provide an excimer laser cavitywherein an absorption edge of an incorporated material limits unwantedlasing.

Another object of the invention is to provide an excimer laser thatenables irradiation of circularly symmetric articles.

Another object of the invention is to provide an excimer laser thatenables irradiation of dispersed media.

Another object of the invention is to provide a solid state laser devicethat allows efficient pumping of a very thin gain volume.

Another object of the invention is to provide a solid state laser cavitythat is self-aligning.

Another object of the invention is to provide a solid state laser devicethat is monolithic.

Another object of the invention is to provide a thin-disk laser cavitythat may be uniformly cooled on both faces.

Another object of the invention is to provide a thin-disk laser cavitythat may be uniformly pumped on both faces.

Another object of the invention is to provide an edge-emitting solidstate slab cavity that may be face-pumped.

Another object of the invention is to provide a solid state slab cavitythat may be edge-pumped.

Another object of the invention is to provide a solid state slab cavitythat deters self-lensing effects.

Another object of the invention is to provide a solid state slab laserthat can be operated with less thermal gradient in the gain material.

Another object of the invention is to provide a solid state slab laserthat can be operated with a lower thermal gradient.

Another object of the invention is to provide a laser cavity mirror witha high laser damage threshold.

Another object of the invention is to provide a laser cavity mirror thatincorporates advantages of both organic and inorganic materials.

Another object of the present invention is to provide a laser cavitymirror with low optical absorption

Another object of the invention is to provide a laser cavity structurethat is inexpensive to fabricate

Another object of the invention is to provide a laser device thatprovides a substantially spherical wavefront for irradiation ofspherical workpieces.

Another object of the invention is to provide a laser device thatprovides

Another object of the invention is to provide a solid state laser devicethat allows efficient pumping of a very thin gain volume.

Another object of the invention is to provide a solid state laser devicethat is self-aligning.

Another object of the invention is to provide a solid state laser devicethat is monolithic.

Another object of the invention is to provide a laser cavity that iscontained within a silicon-on-insulator (SOI) substrate.

Another object of the invention is to provide an optically pumped lasercavity that can be fabricated with integral electronic devices.

Another object of the invention is to provide a laser device that iseasily integrated with silicon-on-insulator (SOI) devices.

Another object of the invention is to provide a means for mode selectionin a laser cavity that utilizes oscillating pump sources.

Another object of the invention is to provide an all-optical switchingmeans that utilizes walking cavity modes.

Other objects, advantages and novel features of the invention willbecome apparent from the following description thereof.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a delimited cross-sectional view of a thin film design for aVHF-BR used in the preferred embodiment.

FIG. 2 is a reflectance characteristic for an VHF-BR coating fabricatedin accordance with the embodiments set forth in FIG. 1, showing normalincidence and tilted reflectance.

FIG. 3 is a sectional top view of the disclosed optical cavity in itsfirst preferred embodiment.

FIG. 4 is a sectional side view of the invention embodied as a sphericalcavity.

FIG. 5 is a sectional side view of the invention embodied as acylindrical cavity.

FIG. 5 is a prior art cavity design incorporating a standard unstableresonator design.

FIG. 6 is a sectional side-view a disclosed cylindrical laser that iscentrally-coupled.

FIG. 7 is a sectional view of a prior art unstable resonator common toexcimer laser designs.

FIG. 8 is a sectional side view of an excimer laser apparatus utilizinga VHF-BR-based cavity that is conically-coupled, wherein thecross-section is taken along the plane formed by axes (9) and (149).

FIG. 9 a is a top view of the excimer laser apparatus utilizing aVHF-BR-based cavity that is conically-coupled.

FIG. 9 b is a bottom view of the excimer laser apparatus utilizing aVHF-BR-based cavity that is conically-coupled.

FIG. 10 is a perspective cut-away view of the excimer laser apparatusutilizing a VHF-BR-based cavity that is conically-coupled, wherein thecross-section is taken along the plane formed by axes (9) and (149).

FIG. 11 is a sectional side view of an excimer laser apparatus utilizinga VHF-BR-based cavity that is cylindrically coupled, wherein thecross-section is taken along the plane formed by axes (9) and (149).

FIG. 12 a is a top view of the excimer laser apparatus utilizing aVHF-BR-based cavity that is cylindrically-coupled.

FIG. 12 b is a bottom view of the excimer laser apparatus utilizing aVHF-BR-based cavity that is cylindrically-coupled.

FIG. 13 is a perspective cut-away view of the excimer laser apparatusutilizing a VHF-BR-based cavity that is cylindrically-coupled, whereinthe cross-section is taken along the plane formed by axes (9) and (149).

FIG. 14 is a sectional side view of a solid state laser apparatusutilizing a VHF-BR-based cavity that is conically-coupled, wherein thecross-section is taken along the plane formed by axes (9) and (149).

FIG. 15 a is a top view of the solid state laser apparatus utilizing aVHF-BR-based cavity that is conically-coupled.

FIG. 15 b is a bottom view of the solid state laser apparatus utilizinga VHF-BR-based cavity that is conically-coupled.

FIG. 16 is a perspective cut-away view of the solid state laserapparatus utilizing a VHF-BR-based cavity that is conically-coupled,wherein the cross-section is taken along the plane formed by axes (9)and (149).

FIG. 17 is a sectional side view of an alternative solid state laserapparatus utilizing a VHF-BR-based cavity that is cylindrically-coupledto a conical reflector and beam-condenser, wherein the cross-section istaken along the plane formed by axes (9) and (149).

FIG. 18 is a sectional side view of an solid state laser apparatusutilizing a VHF-BR-based cavity that is cylindrically-coupled, whereinthe cross-section is taken along the plane formed by axes (9) and (149).

FIG. 19 a is a top view of the solid state laser apparatus utilizing aVHF-BR-based cavity that is cylindrically-coupled.

FIG. 19 b is a bottom view of the solid state laser apparatus utilizinga VHF-BR-based cavity that is cylindrically-coupled.

FIG. 20 is a perspective cut-away view of the solid state laserapparatus utilizing a VHF-BR-based cavity that is cylindrically-coupled,wherein the cross-section is taken along the plane formed by axes (9)and (149).

FIG. 21 is a sectional side-view of a slab-shaped solid state cavitystructure, wherein the cross-section is taken along the plane formed byaxes (304) and (309).

FIG. 22 is a sectional side-view of the slab-shaped solid state cavitystructure, wherein the cross-section is taken along the plane formed byaxes (303) and (304).

FIG. 23 is a sectional top-view of the slab-shaped solid state cavitystructure, wherein the cross-section is taken along the plane formed byaxes (303) and (309).

FIG. 24 is a schematic of a laser apparatus incorporating a multitude ofthe slab-shaped solid state cavity structures.

FIG. 25 is a perspective view of a flexible VHF-BR on a flexiblesubstrate.

FIG. 26 is a perspective view of a planar VHF-BR-based cavity formed ona planar substrate.

FIG. 27 is a perspective view of a planar double-VHF-BR-based cavityformed on a planar substrate.

FIG. 28 is a magnified sectional side-view of the planar VHF-BR-basedcavity.

FIG. 29 is a perspective view if a laser scanning apparatus used forforming a planar VHF-BR-based cavity.

FIG. 30 is a perspective view if a Fabry-Perot-based apparatusalternatively used for forming a planar VHF-BR-based cavity.

FIG. 31 is a sectional side-view of the planar VHF-BR-based cavity ofFIG. 26, further comprising SOI and heat-sinking layers.

FIG. 32 is a sectional side-view of the planar double-VHF-BR-basedcavity of FIG. 27, further comprising SOI and heat-sinking layers.

FIG. 33 is a perspective cut-away view of a laser apparatus comprisingseveral stacked planar VHF-BR-based cavities, taken along central axis(9).

FIG. 34 is a sectional side-view of a planar VHF-BR-based cavity with anintegrated waveguide.

FIG. 35 is a top-view of a planar VHF-BR-based cavity with a multitudeof pump sources.

FIG. 36 is a perspective view of a planar VHF-BR-based cavityincorporating several pump sources and waveguides.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The following description and FIGS. 1-36 of the drawings depict variousembodiments of the present invention. The embodiments set forth hereinare provided to convey the scope of the invention to those skilled inthe art. While the invention will be described in conjunction with thepreferred embodiments, various alternative embodiments to the structuresand methods illustrated herein may be employed without departing fromthe principles of the invention described herein. Like numerals are usedfor like and corresponding parts of the various drawings.

In FIG. 1 is a repeated scheme for the build-up of a high-reflectanceVHF-BR (5). The VHF-BR contains p quarter-wave pairs, each consisting ofa low index layer (14) and a high index layer (15). The substrate (1)provides the surface of revolution onto which the VHF-BR is deposited.Each pair of quarter-wave layers (14) and (15) share a small refractiveindex difference, Δn, which is typically less than 0.1. The number ofquarter-wave pairs, p, will typically be greater than 50 to maintainhigh reflectance. The quarter-wave pairs may be deposited sequentiallyto achieve VHF-BR's containing hundreds of layers. Materials optimallyused will depend upon the spectral region desired for lasing action. Inmany cases the small difference in real refractive index, Δn, may beachieved by making substitutions into the matrix of a parent material.

The precise choice of materials for the high-index and low-index layerswill depend upon the specific optical cavity under consideration. Theindices of the VHF-BR are preferably matched to the index of the gainvolume n_(g), which is also the n_(s) of equation (1.1) so that theindex of the high-index layer, n_(h), is only slightly higher than thatof the gain volume. For example, if the gain volume is fabricated from asilicon dioxide material with a nominal index of 1.46, the high-indexlayer would preferably comprise a material slightly higher in index, forexample n_(h)=1.48. In this particular embodiment, an index of around1.48 may be achieved in a modified or doped silicon dioxide material, orwith various polymer materials, such as polymethyl methacrylate (PMMA),Perspex, or polyvinyl acetate. For this particular index ratio ofn_(l)/n_(h)=0.9865, it is preferable that p is greater than 200, so thata reflectivity of approximately 99% is achieved. Of course, certaincavity designs may suitably incorporate a VHF-BR having less than 99%reflectivity, without departing from the principles and structuresdisclosed herein.

Of course, in accordance with the known properties of a VHF-BR, asmaller difference between n_(h) and n_(l) will result in a smallerreflectance and a greater sensitivity to angle-of-incidence (or finesse)for a given n_(g). It is therefore preferred, to achieve both very highfinesse and adequate reflectivity, that both the ratio n_(l)/n_(h) be asclose to 1 as practicable, and that the number of layer pairs, p, besufficiently large to provide adequate reflectivity for the particularselection of n_(l)/n_(h) being implemented.

The range of values that the above variables can have for realizing thebenefits of a VHF-BR-based cavity will vary considerably, based upon theparticular optical resonator contemplated. For the purposes of thepresent disclosure and resonator designs contemplated herein, a VHF-BRshould have an index ratio in the range, 1>n_(l)/n_(h)>0.95, and p≧50.Preferably, the index ratio is in the range, 1>n_(l)/n_(h)>0.97, andp≧100, and, ideally, an index ratio in the range, 1>n_(l)/n_(h)>0.98,and p≧400. The various combinations of n_(h) and n_(l) p n_(g) that maybe successfully used to provide effective mode-selection in an opticalresonator will be determined by the particular device underconsideration.

For instance, ZrO₂ may be deposited as the parent material by ion beamsputtering, thereby forming one of the quarter-wave layers.Subsequently, the second layer material may then be formed using thesame process, while co-sputtering a second material, such as TiO₂, CeO₂,HfO₂, or NbO₂, from a separate target in the same process chamber,resulting in the second layer being a mixture of the two oxides. As aresult, the refractive index of the second layer may be controllablyrendered slightly higher than that of the first layer; this, through thewell-controlled addition of the second material to a ZrO₂ matrix.

The VHF-BR, as shown in FIG. 1, may also be constructed with additionalthin film structures incorporated for performing additional functions,such as anti-reflection coatings or secondary reflectors, and so forth.However, to achieve the finesse required in the present invention, theVHF-BR design chosen for the cavity mirror must incorporate a highnumber of quarter-wave pair iterations, accompanied by an unusuallysmall index difference, Δn.

In FIG. 2 are reflectance curves, in wavelength λ vs. % reflectance, foran VHF-BR fabricated according to the embodiments in FIG. 1, for lightincident approximately normal to the substrate, wherein the VHF-BR isfabricated to provide highest reflectance at an angle θ_(o)=0°, which isherein equivalent to normal incidence, orthogonal to the VHF-BR surface.The reflectance peak of the VHF-BR at normal incidence, as given by thesolid line (2), is an example of the narrow full-width-half-max (FWHM)achieved with low Δn. The reflectance peaks of FIG. 2 is obtained from aVHF-BR containing ninety pairs (p=90) of the quarter-wave layers, withthe index difference of the pair, Δn=0.04. A topmost high-index layer(49) would typically be deposited to give maximum reflectance, resultingin an odd number of layers (in this case, 181 layers). The dashed line(3) in FIG. 2 is the reflectance peak for the same VHF-BR whenirradiated with light at an angle of 15° from normal incidence. Thespectral shift between the two reflectance peaks of FIG. 2 is found tobe approximately λ₀−λ₁=Δλ=5 nm, while the magnitude of p-polarizationpeak reflectance is also found to drop from 95% to 94%. The magnitude ofthe peak reflectance may be increased through an increase in p; and, aspeak reflectance increases, the latter 1% percent drop becomes anincreasingly decisive factor in determining cavity Q, and modeselection, within a laser cavity constructed in accordance with thepreferred embodiments. The characteristic of FIG. 2 is only fordemonstration, as much narrower reflection peaks may be obtained.

A more narrow, or broad, FWHM (16) may be obtained by varying Δnaccording to the previously described relationships. In addition to thenarrow FWHM, another useful characteristic of this VHF-BR, whenincorporated in the present invention, is the pointed shape of the peak,as this pointed shape allows a more narrowly defined peak reflectance.The utility of these characteristics will become apparent when discussedin conjunction with the embodiments of FIGS. 3-36.

It is not intended that the VHF-BR be restricted to the embodiments ofFIG. 1-2, as the latter embodiments are presented primarily for thepurpose of teaching the invention.

The VHF-BR implemented in a particular embodiment will depend on itsparticular requirements. The VHF-BR may comprise organic or inorganicmaterials, or a combination of both. The design of the VHF-BR may varyconsiderably, as well. For instance, certain layer pairs within theVHF-BR may possess a much higher Δn without appreciably increasing theFWHM of FIG. 2. The thin film materials utilized may possess amorphousor crystalline microstructures; and as such, may be optically isotropic,uniaxial or biaxial, depending upon the precise transmissioncharacteristics desired of the VHF-BR. The VHF-BR may, in someapplications, be designed for peak reflectance at a relatively largeangle of incidence. Various other functions may also be incorporatedinto the VHF-BR coating design, such as an anti-reflection coating, orthe transmission of a particular fluorescence peak.

In FIG. 3, the present invention is shown in its first preferredembodiment. The substrate (1) provides the structure by which thesurface of revolution, with axis of circular symmetry (9), is defined.In the embodiments of FIGS. 3-20, this surface of revolution will beidentical to the interface between the substrate (1) and the VHF-BR (5).The VHF-BR (5), as described in FIGS. 1-2, conforms to this surface ofrevolution and modifies its reflective characteristics. The gain mediumfor the laser is contained within the cavity interior (4), formed by thesubstrate and integral VHF-BR. As such, if a fluorescent event occurswithin the gain medium, its confinement within the cavity is very muchaltered through the incorporation of the previously set forth VHF-BR.The VHF-BR limits the bandwidth of the laser emission, first through theinterference filtering of the normal incidence emission, as practiced inthe prior art. However the circular geometry of the present invention,combined with the high angle-dependence of the VHF-BR, as described inFIGS. 1-2, requires that emission from the fluorescent event alsopropagate within a narrowly defined solid angle, Δθ_(r), if it is to bereflected back into the cavity interior (4). Propagation which occursoutside this solid angle, such as indicated by solid line (6), will beallowed to transmit outside of the cavity interior (4), thereby avoidingthe establishment of laser modes for such off-angle propagation. In thegeometries described, these highly angle-dependent VHF-BR's therebybecome a means of mode selection. The zig-zag line (7) which depicts thedirection of mode propagation is only for demonstration, but indicatesthat the concentration of allowed modes is at or near normal incidence.The precise angle of the dominant mode will be determined by such designconsiderations as the preferred angle-of-incidence, the fluorescencespectra of the gain medium, the type of coupling desired, etc.

In the laser cavity structure of the present invention, confinement ofthe laser modes to paths that are at or near to normal incidence allowsseveral unique coupling configurations. One such configuration is shownin FIG. 3, wherein laser radiation is coupled from the laser byintroducing the media to be processed into the center of the lasercavity. This may be accomplished through implementation of a tube (8),which separates the gain medium from the process media passing throughthe tube interior, thereby providing a process volume within the cavity.The latter embodiment will be particularly effective in the processingof media that possess low absorption cross-sections, such as gases andvapors. Alternatively, the central coupling structure designated by thetube (8) may instead contain a cone-shaped optical element forextraction of laser light from the center of the cavity as has beendescribed in numerous papers and patents of the prior art.

The cross-sectional figure of the cavity mirror may be designedvariously, dependent upon the type of gain medium and lasing actionrequired. In FIG. 4, the surface of revolution possesses across-sectional figure with a radius of curvature equivalent to that ofthe surface of revolution as viewed from the top in FIG. 3, therebyrendering it a spherical section. In this embodiment, laser emission isconfined to propagate through a volume (17) located at the center of thespherical mirror, intersected by the axis of circular symmetry (9),thereby allowing an unusually high power density within this smallvolume.

It should also be noted that the embodiments of FIGS. 3-4 do not requirethat the described spherical cavity laser be restricted to anyparticular major spherical section. In fact, the cavity structuresectional view of FIG. 4 may as easily describe operation of a cavitystructure that is not truncated at all, so that the cavity is a completesphere. Also, the VHF-BR described herein may, in many circumstances, bedeposited on the external surface of the substrate, therein defining thesurface of revolution. In these latter circumstances, the substratewould reside within the cavity interior, and hence would need to bequite transparent to the desired wavelengths. Such a case might be whenthe required surface of revolution is the external surface of a sphere,which may be composed of a laser glass or crystalline material.

Another embodiment of the present invention is presented in FIG. 5, inwhich the cross-sectional figure of the surface of revolution—again,identical to the VHF-BR/substrate interface—is straight, therebyrendering the surface of revolution a cylinder. The cylindrical shape ofthe laser cavity structure in the latter embodiment serves todemonstrate an added utility that is realized with the incorporation ofthe described VHF-BR's. Unlike the cavity geometries of the prior art,linear and other, which use relatively low-finesse reflectors, thepresent invention allows the stability associated with a particularcavity mirror selection to be increased. Whereas flat (or cylindrical)cavity mirrors will typically support parasitic “walk-off” modes whichcan decrease the overall Q-factor of the laser cavity, these same modes,such as exemplified by propagation direction (6) in FIG. 5, will bediscouraged due to the low reflectivity of the cavity mirrors at theseangles.

The laser cavities described in the present invention may comprise gas,solid, or liquid gain media, and may be pumped by any of the compatiblemethods described in the art. Also, the present invention allows for aunique method of optical pumping.

It should be noted that, in embodiments of the present invention wherethe laser cavity is fabricated with a disk-like aspect, thermalstability is typically more easily obtained than in other lasercavities. This latter advantage is due to the ability to effectivelyheat-sink the cavity through its planar sides—as indicated by dashedlines (18) in FIGS. 4-5—as these surfaces need not be transparent. Infact, these surfaces can possess any of a number of reflecting,absorbing, or scattering characteristics, depending on the application.The ability to heat-sink these cavities can be particularly important inthe case that the gain medium is solid state. Heat-sinking, in such acase, may also be performed effectively through the cavity mirror, aslong as the outer layers of the cavity mirror are specified so as toprevent any possible TIR of unwanted laser wavelengths. If the lasercavity structure of the present invention is to be operated in anambient medium which possesses a refractive index, n_(A), substantiallylower than n_(G), then an absorbing and/or scattering layer ispreferably utilized externally to the VHF-BR. This latter use of anabsorbing and/or scattering layer serves to prevent specular reflectionof unwanted cavity emissions back through the VHF-BR to re-enter thegain volume. Such measures could be implemented in the case that thegain medium is solid state.

In the case that optical radiation is to be coupled from the center of acircular VHF-BR-based cavity that utilizes a cylindrical surface ofrevolution as the surface on which the VHF-BR is formed, such extractionmay be performed through use of a conical reflector, in FIG. 6. Theconical reflector is preferably a metallic reflecting surface (137),such as an aluminum film, that is formed on the conical surface of theconical reflector structure (136), which is preferably formed from aneasily heat-sinked material, such as copper, aluminum, silicon carbide,etc. In the embodiments of FIG. 6, the conical reflector is aninter-cavity mirror that folds propagating light toward an outcouplermirror (138), which is a semi-reflecting planar mirror possessing anannular clear aperture sufficient for outcoupling the incident annularbeam reflecting from the conical reflector. Further embodied is theimplementation of beam condensing means that comprise a nominallytelescopic optical assembly. The beam condensing means of the presentinvention includes a primary reflector element (140) and a secondaryreflector element (139), so that light coupled from the outcouplermirror is incident onto the reflective surface of the primary element,the light reflected by the primary element to be incident onto thesecondary element, the light reflected by the secondary element to exitthe central aperture (141) of the primary element, thereby providing anoptical beam that propagates along central axis (9). The opticalassembly comprising the primary and secondary element may be specifiedvariously, but is preferrably an afocal Cassegrainian design, whereinthe primary surface is a paraboloidal-concave surface, the secondaryelement provides a hyberboloidal-convex reflecting surface, and the tworeflective elements are positioned to provide a parallel beam that exitsthe central aperture. Such designs are known in the prior art, and arediscussed in Astronomical Optics, by Daniel J. Schroeder, 2^(nd) Ed.,Academic Press, e.g., pg. 200, included herein by reference. It may bereadily seen that other nominally telescopic designs may bealternatively used to provide either parallel or focussed exit beams,such as other Cassegrainian, Newtonian, or Gregorian optical assembliesspecifically designed for focussing or condensing parallel light beams.

The VHF-BR-based cavity structure of FIG. 6 is particularly advantageouswhen utilized in conjunction with excimer-based gain media, due to the“built in” nature of the population inversion in excimers. The absenceof a ground state in excimer gain media results in the propensity formany modes to readily experience gain in conventional stable resonators,so that various unstable resonators are utilized to limit the number ofresonator modes that may experience high confinement by the unstableresonator mirrors. Such unstable resonators, in FIG. 7, generallyutilize at least one convex mirror (24) that is incorporated in a linearresonator opposite the outcoupler mirror (23), so that a relativelylimited number of modes are present in the resulting laser beampropagating along the optical axis (25). Such unstable resonators arethus an inefficient means of providing laser energy, since themode-discrimination means requires continual loss of energy to walk-offmodes. Thus, the VHF-BR-based cavity structure provides an attractivealternative to such unstable resonator designs, since the VHF-BRprovides for high confinement of normal-incidence propagation across anextended aperture, namely, within the plane of oscillation in FIG. 3,when the VHF-BR is designed to provide maximum reflectance for the peakemission wavelength at normal incidence. At the same time, the circularVHF-BR-based cavity design provides sharply decreasing confinement formodes that rely on non-normal incidence, so that the off-axis modes thathave very low cavity Q in the prior art unstable resonator of FIG. 7will similarly have a low cavity Q in the circular VHF-BR-based cavity,whereas the desired normal and near-normal incidence radiation will behighly confined across the extended aperture of the VHF-BR surface. Asdisclosed earlier, the VHF-BR may also include materials having anabsorption edge to further decrease reflection of unwanted shorterwavelengths that may otherwise be reflected at non-normal incidence.

An excimer-based laser apparatus that incorporates the advantages of acircular VHF-BR-based cavity is accordingly disclosed herein. In thepresent disclosure, laser apparatus that rely upon excimer transitionsfor a characteristic emission will be termed as excimer-based lasers,whereas the various gases utilized for providing gain in excimer-basedlasers will be referred to as excimer-based gain gases or media.

TABLE 3 Excimer transitions of selected gases Kr₂ Xe₂ ArF (B—X) KrCl(B—X) XeCl (B—X) 146 nm 172 nm 193 nm 222 nm 308 nm

The preferred embodiments of such an excimer laser apparatus provide foran annular gain volume (125), that is concentric to a circular reflectorcomprising a substrate (1) providing the surface-of-revolution andVHF-BR (5) formed thereon, in FIGS. 8-13. In accordance with theembodiments of FIG. 6, the present excimer laser also incorporates aconical reflector structure (136) providing a conical reflector surface(137). The various components of this excimer laser apparatus arepreferably positioned and sealed between first annular flange (144) andsecond annular flange (145). The flanges incorporate a circular array ofgas inlets (154) and a circular array of gas outlets (155) that areconcentric to the circular cavity.

Prior art excimer-based lasers typically utilize opposing linearelectrode configurations for providing electron discharges or otherforms of discharges for exciting the excimer-based medium that is passedbetween the linear electrodes. The disclosed VHF-BR-based excimerinstead utilizes opposing first annular electrode (151) and secondannular electrode (152), so that the gain volume (125) residing betweenthe two annular electrodes is accordingly annular as well. An annularelectrode insulator (150) provides insulation between the electrodes anflange housing.

The embodiments of FIGS. 8-13 are particularly advantageous for reasonsother than the mode discrimination provided by the circular VHF-BR-basedcavity. The circular geometry of the annular electrodes and adjoiningannular discharge region (roughly, the gain volume) provide forinherently higher operational stability than is provided by conventionallinear electrodes. This higher stability is owed to the absence of“end-effects” that are intrinsic to any linear discharge, whereinproperties of the discharge—such as the electrical field, conductivityof the discharge gas, electron-density, gas composition, etc.—aredifferent in the vicinity of the linear electrode's either end thenalong the electrodes middle region. This inhomogeneity begets localchanges in the surface chemistry and electrical properties of theelectrode and nearby surfaces, so that degradation of homogenousproperties in the linear electrode discharge must result. Such problemsare by-passed in the presently disclosed excimer laser, since no suchasymmetry or end-effects exist in discharges sustained between theannular electrodes. The high symmetry of discharges supported by theannular electrodes is further improved by the radially symmetric gasinlets and gas outlets, which allow for a radially symmetric gas flowand pressure gradient to exist across the annular electrodes.

All components of FIG. 8 are radially symmetric about the central axisof symmetry (9), except for the electrode power leads (153).

In an alternative embodiment to the telescopically coupled excimer laserin FIGS. 8-10, a similar excimer laser design may incorporatecylindrical coupling means, rather than the earlier disclosed conicalcoupling means. In the present cylindrically coupled embodiment, inFIGS. 11-13, the excimer is centrally coupled via a cylindricaloutcoupler (156) that preferably comprises a cylindrical glass windowsuch as fused silica (e.g. Optosil™) that is transparent to the desiredlaser emmission. In the case of vacuum ultraviolet emitting gain mediais preferably constructed from calcium fluoride or other appropriatematerial. The cylindrical outcoupler provides a semi-reflecting surfacefor retaining and transmitting cavity light, preferably by means of aoutcoupler coating (157) deposited onto the outer cylindrical surface ofthe cylindrical outcoupler. The first flange (144) and second flange(145) perform similarly as before, except that a process port (160) iscentrally provided.

Such cylindrically coupled embodiments of the disclosed excimer laserprovide a useful means for irradiating materials or a work pieces (148)that are passed through the process space (147) of the laser. Suchmaterials that may be advantageously processed by such treatment includeoptical fiber preforms, gases, vapors, wires, pipes, etc. Such a processspace may also find use for deposition process wherein a depositingvapor is first passed through the process space to be ionized ordissociated. In some cases wherein a process gas or vapor is to beprocessed in the process space, it may be possible to forgoimplementation of the outcoupler coating (157), or other means forproviding an outcoupler characteristic to the cylindrical outcoupler,which may then simply be a transparent window, so that such gases orvapors may then be passed through an intercavity process space.

It may also be seen that the circular VHF-BR-based cavity is useful forcreating a mode structure that is also circularly symmetric, so that anessentially annular field intensity distribution (159) may be created inthe process space. Such an annular field intensity may be useful forsuch applications as providing symmetric modification with in theinterior of a glass fiber or preform, or in performing chemical vapordeposition at the surface of such cylindrical workpieces. Of course, anextra-cavity conical reflector could be readily implemented to redirectlaser emissions in the process space into a parallel beam, similarly toFIG. 6. In embodiments of the disclosed excimer laser that utilize aVHF-BR that is deposited on the outside of a circular window thatprovides the surface of revolution, it clearly necessary that the windowalso be composed of a material that is transparent to the desired laseremission, similarly to the cylindrical outcoupler.

The embodiments of FIG. 6 are also applied for particular advantages inlaser apparatus and optical amplifiers based on solid state media. Inthese embodiments, in FIGS. 14-20, a solid state medium is formedsimilarly to the circularly symmetric cavity of FIG. 6.

Accordingly, the embodiments of FIGS. 14-20 incorporate a disk-shapedsolid state medium (201) that has an annular aspect. The discloseddisk-shaped solid state medium possesses a first outer edge providingthe specular surface-of-revolution for forming the VHF-BR (5) thereon.This outer surface of revolution is again preferably cylindrical. Thedisk-shaped media of FIGS. 14-20 also possess a second surface ofrevolution provided by the inner edge of the annular disk medium. In theembodiments of FIGS. 14-16, this second surface-of-revolution of thedisk medium is a conical surface with a metallic reflector deposited onthe conical surface, thereby providing the conical reflective surface(137) of previous embodiments.

The conical surface is preferably temperature-controlled by atemperature-controlled central optical structure (211), which has aconical surface to provide uniform contact to the reflective coatingdeposited on the conical surface of the disk medium. The central opticalstructure (211) is preferably a monolithic structure that alsoincorporates the reflective surface of the secondary reflector (139).The outcoupler mirror (138) in the present embodiment is a planar mirrorformed as an annulus, as in earlier embodiments, except that the annularoutcoupler mirror may readily be formed as a semi-transparent coating,rather than a separate optical element, in the present embodiment.

The solid state media of FIGS. 14-20 are preferably optically pumped byoptical pumping sources (204), which are incorporated in an edge-pumpingconfiguration in FIGS. 14-16. In this embodiment, optical pump sourcesare housed in a pump housing structure (207) that also contains focusingmeans (208) for directing pump light into the disk medium (201). TheVHF-BR (5) is transmissive to the pump source radiation.

Cooling of the disk-shaped solid state medium of FIGS. 14-20 isperformed by direct cooling through the planar faces (214), so that suchcooling of the disk faces may provide control of the temperature in thedisk-shaped gain medium, thereby providing a uniform temperaturedistribution in the gain medium. Such cooling of the faces isincreasingly effective as disk media are fabricated with increasinglysmall aspect ratios, and smaller thicknesses, d, are utilized. Thecooling of the faces is performed by first and second cooling plates(205 & 206) which may alternatively house additional optical pumpingmeans. The cooling plates are preferably bonded to the disk (201) via ahigh thermal conductivity layer at the interface between the plate anddisk. This bonding layer may be formed on the disk faces (214), where itis deposited over an optical layer that is reflective to the pumpradiation, as well as preferably absorbing of the propagation ofunwanted cavity modes, thereby frustrating TIR of such unwanted modes.

An alternate embodiment to the disk laser apparatus of FIGS. 14-16utilizes a disk-shaped solid state gain medium with an annular aspect,except that the second, inner, surface-of-revolution is a cylindricalsurface, in FIG. 17. As previously, a outcoupling mirror (138) isutilized, in FIG. 17, that comprises a coating deposited over thecylindrical inner surface. As in the embodiments of FIGS. 14-16, acentral optical structure (211) provides the secondary reflector (139),but also provides a the conical reflector surface (138) as anextra-cavity mirror. As before, beam condensation means are provided viathe primary reflector (140) and secondary reflector, with opticalhousing means (142) attached to the first cooling plate.

Another embodiment of a solid state laser apparatus utilizing aVHF-BR-based cavity and disk-shaped gain medium, in FIGS. 18-20,utilizes the same annular disk medium of FIG. 17, except that it isface-pumped by cooling plates that incorporate optical pumping sourcestherein.

The disclosed VHF-BR used for mode-discrimination in solid state mediacab also be use in solid state media that do not possess asurface-of-revolution. In another embodiment, the VHF-BR is made areflecting mirror in polygonal solid state media having a “slab”characteristic. Polygonal solid state cavities that are seen to benefitmost from use of a VHF-BR cavity mirror are those in which the mediumhas opposing faces that have a dimension substantially longer than otherdimensions of the medium, and in particular, considerably longer thanthe dimension defined by the thickness between the two faces. Such amedium is used in conjunction with a VHF-BR, in FIG. 21, to provide ameans for reducing propagating modes to those having propagationdirections orthogonal to the VHF-BR. A solid state slab module (300) ofthe preferred embodiments is fabricated using a rectangular gain medium(301). The rectangular gain medium has, in addition to a relativelysmall dimension along thickness axis (303) between its larger parallelfaces, also two parallel optical faces between which propagation isorthogonally reflected, such propagation parallel to lateral axis (309).The optical faces are thus those parallel to axes (303) and (304). Oneface, which is terminated with the VHF-BR, provides a high reflectancemirror for the propagation, whereas the other surface is an outcouplingmirror (302), or in the case of an optical amplifier, the outcouplingmirror may be substituted by an antireflection coating. Due to theVHF-BR, unwanted propagation (315), which propagates non-orthogonally tothe VHF-BR, is allowed to exit the cavity, so that only orthogonalpropagation is effectively pumped. In addition top and bottom faces ofthe cavity may be coated with an absorbing layer (310) that additionallyreduces any TIR of the unwanted propagation.

This rectangular slab geometry allows for the larger faces, parallel toaxes (304) and (309), to be efficiently cooled, in FIGS. 22-23 bycooling structures (307) that also provide for optical face-pumping ofthe slab. Such simultaneous pumping and cooling of the large faces maybe provided in a very uniform fashion on both sides of the slab, sinceno cavity radiation is to be coupled form these larger faces. Inaddition, intermediate layers (308) may be utilized for specificallyabsorbing unwanted cavity propagation, high thermal conductivity tocooling structures, and transmission of pump radiation,

In the case that the outcoupling mirror of FIGS. 21-23 is substituted byan antireflection coating, and the module (300) comprises an opticalamplifier, then a multitude of such amplifiers may then be opticallycoupled together to form a high-power laser, in FIG. 24. Such couplingmay be realized through 45° beamsplitter mirrors (321) and beamsplitterhigh-reflectance mirrors (320), the combination being readily availablein the form of beamsplitter cubes. A Brewster window (318) may beoptionally used for polarization control. A high reflectance mirror(317) and outcoupler mirror (319) complete the laser cavity, providingconfinement and outcoupling of the optical beam (322).

A polymer-based VHF-BR, in FIG. 25, provides another embodiment whereinthe VHF-BR (5) is a flexible structure that is, in one embodiment,deposited onto a flexible plastic film (401). Because of the low opticalextinction possessed by many polymers, such polymers are attractivematerials from which to fabricate the VHF-BR. A characteristic ofpolymers that can inhibit the high finesse characteristic of thedisclosed VHF-BR, however, is birefringence. While very substantialbirefringence is found in thin film polymers that are produced by thestandard manufacturing processes of polymer film, such opticalanisotropy is much more easily avoided in vacuum-deposited polymers,since mechanical, linear stretching is not required. In addition,orientation of polymer chains within vacuum-deposited polymer films maybe further manipulated through a variety of means, including utilizing alow index oxide—such as SiO₂—as either the low-index or high-indexlayer. Such use of an underlying material to provide a moleculartemplate by which a subsequently deposited polymer film ispreferentially randomized or oriented will be considered herein as asubset of epitaxial means. Other epitaxial means may be provided by useof a particular polymer or surface treatment. Other means formanipulating the orientation of polymer films as they deposit mayinclude exposure of the deposited film to various plasma, electron beam,UV exposures, or electrostatic forces during deposition.

In one alternative embodiment, the polymer-based VHF-BR (eitherpolymer-polymer or polymer-oxide) may be deposited onto a flexible orrigid substrate that is previously treated with a release layer, sothat, in this particular embodiment, the VHF-BR may subsequently beseparated (released) from the substrate, so that the VHF-BR may beutilized in subsequent applications in which it is preferable to havethe substrate absent.

The releasable VHF-BR is particularly suited for acquiring reliablesurface properties from a polymer-based VHF-BR that is desired for itsflexible properties, since such a VHF-BR may be first deposited onto anextremely smooth and optically precise substrate that possesses a moreprecisely controlled surface than is typically possible to achieve inflexible substrates (e.g., Mylar®). Release agents may comprise amaterial that can be preferentially etched, a soluble material, amaterial with low-surface energy, or any other appropriate releaseagent.

The polymer-based VHF-BR, in FIG. 25, may then be utilized in a widevariety of applications, including but not limited to lamination of thedisclosed optical cavity, short-pass filtering, decorative applications,flexible electronics, flexible displays, etc. In the event that thepolymer-based VHF-BR is utilized in laser construction, thepolymer-based VHF-BR could be bonded to the surface of, for example, asolid-state laser medium to provide a mode selecting cavity reflector asdiscussed previously. Such application would not require a low-indexcavity in certain slab-cavity lasers.

The thin film VHF-BR of the disclosed plastic laser cavity may comprisealternating layers of organic material; or, alternatively, alternatinglayers of an organic material and an inorganic material. Also, while theVHF-BR may be deposited onto a glass substrate, as discussed earlier, itmay alternatively be deposited onto a plastic film, so that theVHF-BR/film may then be applied over a separate substrate having thesurface of revolution. In the case that the VHF-BR comprises alternatinglayers of a polymer and an inorganic oxide, the surface morphology ofthe deposited oxide film may be made to possess either a sheer, smoothmicrostructure, or it may be rough microstructure. While the smoothmicrostructure is typically much more commonly specified for lasercoatings, the relatively rough microstructure may be found advantageousin certain circumstances wherein more diffuse boundaries between layersof different refractive index are useful, such as when rugate propertiesare desirable, or when the VHF-BR is required to provide increasedflexibility or thermal cycling.

Because the disclosed cavity structures of previous embodiments areformed by a single surface of revolution, fabrication of the VHF-BR maybe performed in a highly precise manner. The deposition ofwell-controlled and homogenous layers is accomplished by rotating thecircular substrate so that its surface of revolution is continuouslypassed by the deposition source at a rapid speed. The rapidly rotatingsubstrate is then provided a very even coating by each vapor source. Asis taught in the prior art of vacuum deposition of polymer thin films,formation of the polymer thin film is performed via first depositing alayer of a monomer or oligimer and, second, curing the layer topolymerize into a polymer film. Water or gas cooling of the substratemay be performed to more reliably determine the condensation behavior ofthe organic vapor.

The curing for polymerization of the monomer/oligimer layer may beperformed by the various methods discussed in the polymer thin film art,including ultraviolet, electron beam, and plasma curing. Such curingmeans may be provided by commercially available modular sources that maybe mounted in the vacuum-processing chamber.

In the case that the VHF-BR is to be deposited onto a thin plastic film,such as a Mylar® film, the flexible substrate may be mounted onto acooled rotating drum, as is commonly practiced in the web coating art.In some cases, the VHF-BR may possibly be deposited in a continuousroll-to-roll web-coating configuration, though, the VHF-BR is, in thispreferred embodiment, deposited onto a single, rapidly spinning drum, sothat layer thickness and homogeneity may be precisely controlled.

Because the flux of uncured monomer/oligimer to the substrate is quitesmall, very little heat removal capacity is required. Either gas orliquid cooling of the substrate may perform substrate cooling, asrequired. It is preferred that an optical monitor monitors thedepositing VHF-BR, preferably both in reflectance and transmission, sothat slight changes in intensity may be monitored in the beginning ofdeposition.

In the case of a previous embodiment, wherein the cavity contains agaseous or liquid gain medium, the surface of revolution may reside onthe interior surface of a rigid circular substrate, as in FIGS. 3-4. Inthis latter case the deposition may accordingly be performed with thevapor source depositing material on the rotating substrate from theinside, wherein the substrate must then be secured by its externalsurface

The monomer vapor source may be any monomer vapor source of the priorart, including but not limited to flash evaporation, boat evaporation,Vacuum Monomer Technique (VMT), polymer multilayer (PML) techniques,evaporation from a permeable membrane, or any other source foundeffective for producing a monomer vapor. For example, the monomer vapormay be created from various permeable metal frits, as previously in theart of monomer deposition. Such methods are taught in U.S. Pat. No.5,536,323 (Kirlin) and U.S. Pat. No. 5,711,816 (Kirlin), amongst others.

For formation of the VHF-BR structures, the vacuum deposition sourcesmay be specified variously, depending on which of the variousembodiments of the invention discussed are to be formed. For formationof the VHF-BR onto a polymer, whether the polymer is the flexiblesubstrate or an underlying cured polymer film, the inorganic layer isfirst deposited by an inorganic vapor source, which, in the firstpreferred embodiment, is a magnetron sputter source as is commonly usedfor deposition of inorganics in the prior art. The magnetron may be ofthe unbalanced magnetron design for providing sufficient activation ofthe deposited inorganic during deposition. For formation of the VHF-BR,the magnetron source may be operated under a wide variety of operatingconditions, depending on the material being deposited, the condition ofthe underlying substrate, the substrate temperature, partial pressuresof reactive gas, total operating pressure, magnetron power, distancebetween the magnetron sputter source and the substrate, etc.

Deposition means for the inorganic material may be any method used forvacuum deposition, including but not limited to chemical vapordeposition, plasma enhanced chemical vapor deposition, sputtering,atomic-layer deposition (ALD), electron beam evaporation, electroncyclotron resonance source-plasma enhanced chemical vapor deposition(ECR-PECVD) and combinations thereof.

The ability to provide a high cavity quality Q for a very limited solidangle of reflection, Δθ_(r), as provided by the operational principlesset forth herein, VHF-BR-based optical cavities may be fabricated withgain volumes possessing an unusually small aspect ratio. Such aspectratios may correspond to essentially a cavity that has a thicknesscorresponding to that typical of optical fiber cores, and accordingly,may be so thin that d is on the order of the gain wavelength.

In one embodiment of the invention, circular VHF-BR-based opticalcavities with very thin aspects are formed in thin layers (1 um<d<1000um) of dielectric material formed on a supporting planar substrate. Sucha dielectric cavity layer may be formed by the various vapor depositionmethods that are commonly used for forming dielectric layers onsubstrates, such as chemical vapor deposition, sputtering, evaporation,etc. The dielectric material may be any suitable dielectric, but ispreferably high-purity silicon dioxide. Whereas earlier embodiments ofthe disclosed VHF-BR-based optical cavities utilized VHF-BR's whereinthe low-index layers and high-index layers are preferably formed byvapor deposition of the layers onto a substrate having the surface ofrevolution, the present embodiment preferably utilizes localizedmodification of a single dielectric layer on a planar substrate to formthe concentric rings of the circular VHF-BR within the dielectric layer.Accordingly, the high-index layers and low-index layers of the VHF-BRare formed, in this embodiment, to provide a much smaller cavitythickness, d, than is readily accomplished in earlier embodiments.

The planar substrate (501) may comprise various materials or multilayerstructures, but is preferably a polished silicon wafer of high flatness(e.g., λ/4 at 530 nm) that is subsequently processed to have anadditional, preferably high-index, substrate cladding layer (502) formedover the planar surface, in FIG. 26, so that the planar surface isterminated with a layer that forms a high-index cladding layer for thedielectric cavity layer (503) to be subsequently formed thereon. Theterm “high-index” will refer to the refractive index of any opticalmaterial possessing a higher refractive index than that of thedielectric cavity layer. The dielectric cavity layer (503), preferablysilicon dioxide, is then formed on the planar surface of the planarsubstrate. A circular VHF-BR (504) is then formed within the dielectriccavity layer preferably by using local modification to form thehigh-index layers of the of the VHF-BR. Once the VHF-BR is formed, aplanar VHF-BR-based cavity is thus formed, wherein a cavity interiorvolume (505) is formed by the unmodified portion of the dielectriccavity layer (503) that resides within the circular VHF-BR. This cavityinterior volume is, in addition, preferably transformed into a gainvolume by diffusion of a suitable gain material into the heretoforeunmodified cavity interior volume. Such a suitable gain material maycomprise various rare earth ions, such as Er, Nb, Yb, Pr, etc. Doping ofthe cavity interior volume by rare earth ions may be achieved by variousmethods taught previously, such as by deposition of a thin layer of therare earth material onto the portion of the dielectric cavity layer(503) comprising the cavity interior volume (505), followed by a heattreatment to diffuse the rare earth species into the cavity interiorvolume, so that a gain volume is formed in the dielectric cavity layer.Alternatively, ion-implantation may be used for attaining similarresults. In some instances, it may be preferable to form the cavityinterior volume initially as a doped gain material during the formationof original dielectric cavity layer (503). Henceforth, the cavityinterior volume (505) is considered herein as identical to a gain volumein embodiments wherein gain is desirable.

In FIG. 27, planar double-VHF-BR-based cavity is disclosed as analternative embodiment, wherein a second VHF-BR (506) is incorporatedinto the dielectric cavity layer. In this embodiment of a double-VHF-BR,the gain volume (505) is then formed as an annular region in thedielectric cavity layer, so that either of the two VHF-BR's of FIG. 27may be fabricated to provide outcoupling characteristics, either bybeing fabricated with less reflectivity (smaller p, or smaller Δn), orby further processing the dielectric layer to provide a linear-pathoutcoupler (507) through the VHF-BR. The linear-path outcoupler (507)may comprise a linear path that is formed by making the index modulation(or, Δn) of the VHF-BR less along the linear path. This may readily beperformed through selective use of known UV exposure methods describedand referred to herein. An extra-cavity region (508) is thus formed inthe cavity structure, in FIG. 27, which may be utilized to positionvarious structures for subsequent use of cavity light emitted into thiscentral region.

The dielectric layer is modified by index-modification means to containa VHF-BR-based cavity. This modification is achieved by utilizingmethods and apparatus that provide a localized modification of therefractive index of the dielectric layer, usually comprising a slightincrease over the refractive index of the pre-existing dielectric layer.

A magnified section of the planar VHF-BR-based cavity as embodied inFIGS. 26-27, in FIG. 28, shows the planar dielectric cavity layer (503)is modified to form the consecutive concentric layered regions oflow-index layers (14) and high-index layers (15) needed to form theVHF-BR. Modification is preferably performed by ultraviolet irradiationby an ultraviolet laser beam (511), which is preferably directed only tothose regions of the dielectric cavity layer where the high index layers(15) are to be formed, wherein the index of the irradiated region isslightly increased. Similar methods as those discussed are well-knownand practiced commercially in the prior art for modification of SiO₂fibers into fiber gratings, such as in U.S. Pat. Nos. 5,732,170 and6,125,225, which are included herein by reference.

FIG. 29, a laser scanner set-up is used to provide modification of thedielectric cavity layer (503) to form the high index layers (15) in FIG.28. A UV laser scanner (510) provides a scanned laser beam (511) that isdirected onto the portion of the dielectric cavity layer in which theVHF-BR is to be formed. The ring-shaped pattern of the circular VHF-BRis formed by rotating the planar substrate/cavity layer during lasermodification via a rotating wafer chuck (512), so that the laser scanneris only required to advance the laser beam in a linearly to provide therequired circular pattern.

Alternatively, the dielectric cavity layer may be selectively coatedwith a layer of dopant material, such as Germanium, in concentricring-shaped regions of the dielectric cavity layer where the high indexproperties are needed to form the VHF-BR. The layer of dopant materialmay be patterned via photoresist-based lithography methods common insemiconductor manufacturing. Exposure of the photoresist with theannular pattern corresponding to the high-index layers of the desiredVHF-BR may alternatively be accomplished by means of the exposuremethods previously outlined for directly modifying the SiO₂ layer withUV light. After obtaining the concentrically patterned layer ofGermanium, or other suitable dopant, on top of the SiO₂ layer, theresulting assembly may then be annealled so as to diffuse the dopantinto the SiO₂ layer, thereby forming the high-index layers of desiredVHF-BR.

In FIG. 30, a Fabry-Perot cavity is utilized to image the ring patternof the desired VHF-BR, wherein an UV point light source (524) is used toprovide a spherically expanding wavefront. A first Fabry-Perot plate(521) and second Fabry-Perot plate (522) form a resonant cavity forcreating optical interference in the form of a concentric pattern ofcircular virtual fringes, which is made into an imaged pattern of realfringes on the dielectric cavity layer (503) by optical imaging means(523) that comprise conventional optical assemblies used for imagingvirtual fringes into real imaged fringes. An optical stop (525) may beused to define the region on the substrate (501) that is to bepatterned. Such imaged fringes may be used to provide modification ofthe cavity layer in the same manner as the scanned UV light of theprevious embodiment.

In one embodiment, the wafer-based optical resonator can be furtherprocessed to provide an additional layer of silicon over the dielectriclayer in which the VHF-BR resides. This silicon layer is preferably amono-crystalline layer that is bonded to the dielectric layer by meansof silicon-on-insulator (SOI) fabrication methods described and used forproducing SOI substrates. A preferred SOI process is SOITEC's Smartcut™and related processes, wherein the silicon layer is formed by opticallycontacting the silicon layer to the dielectric layer. SOITEC's processis an example of a preferred process for producing the currentlyembodied SOI substrate, since it is well-known to form a suitablesilicon layer over a wide variety of dielectric materials, which may ormay not have an underlying substrate composed of silicon. Such SOIprocessing methods also provide for optically contacting the Si caplayer to the insulator layer in ways that do not require excessivetemperatures that may distort the VHF-BR. Once the silicon layer isformed by a suitable SOI process, semiconductor circuitry and devicesmay then be formed within and adjacent to the silicon layer as normallyperformed in the semiconductor industry. Thus the disclosed VHF-BR-basedresonator may be incorporated into an SOI substrate that may be appliedto other SOI applications. Various devices that may be fabricated in thepresently discussed SOI substrate include semiconductor-based emitterssuch as light-emitting diodes that may be utilized for pumping of theunderlying gain volume.

In FIG. 31, a SOI substrate incorporating a VHF-BR-based cavity isintegrated into a SOI substrate that includes a second clad layer (530)and a silicon capping layer (527) that is preferably single-crystalline,but may be polycrystalline or amorphous. The structure in FIG. 31 alsoincludes cooling means (531) that is a planar layer (or plate) thatprovides high thermal conductivity to a heat-sinking means, such asthermoelectric or fluid cooling. The structure may alternatively providea central bore (507) wherein central outcoupling structures may bepositioned.

In FIG. 32 a double-VHF-BR-based cavity may similarly be incorporatedinto an SOI substrate. In this embodiment, outcoupling of light from thecavity may be realized through either the inner VHF-BR (506) or theouter VHF-BR (504). Coupling from the outer VHF-BR may be achievedeither by the modified outcoupler (532) of FIG. 27, or by forming theouter VHF-BR with fewer layers than the inner VHF-BR, so thattransmission is greater through the outer VHF-BR.

In the present embodiment, a silicon wafer may be utilized to form ahigh-purity SiO₂ layer by means of thermal oxidation, as is commonplacein semiconductor manufacturing.

The planar substrate may alternatively be means for eliminating unwantedlight propagation within the cavity by allowing such unwantedpropagation to exit through the planar substrate. Alternatively, a theplanar substrate may comprise materials that are transmissive to adesired pump wavelength, so that the VHF-BR-based cavity can be pumpedthrough the planar substrate.

It should be recognized that the planar substrate (501) may be suitablymade from of a variety of materials. Such materials may comprise anamorphous, polycrystalline, or single-crystalline material. Thethin-cavity substrate may be either transparent or opaque to variousradiation utilized in the disclose laser cavity, such as the pump orgain wavelengths. For example, the planar substrate may comprise a fusedsilica wafer, with a high-index cladding layer preferably formed overits planar surface. The use of a fused silica wafer as the planarsubstrate is an alternative embodiment that is particularly suited forthe use of the disclosed VHF-BR-based cavity in stacked arrays ofoptically pumped amplifiers, wherein the amplifiers comprise thecentrally-coupled VHF-BR-based amplifiers of FIG. 31 that areincorporated into the optical path of a laser. Such an embodiment isprovided, in FIG. 33, through the use of a retro-reflection structure(534) comprising a fused silica cylindrical rod that incorporates aplurality of retroreflective conical reflectors (138), wherein theconical reflectors are not metallic as in previous embodiments, but aresemi-reflecting surfaces comprising all-dielectric materials, so thatthe individual VHF-BR-based cavities, in FIG. 33, are optically coupled.The ends of the cylindrical rod are preferably coated with outcoupler(532) and high-reflectance mirror (533), so that a laser cavity isformed by the coupled VHF-BR-based amplifiers.

Various precision optics suppliers can provide conical optics, in FIG.33. The individual conical reflectors in FIG. 33 are preferably formedby first fabricating elements with mating conical surfaces, second,coating one mating face of the mating elements with a dielectricreflector, and, third, mating the mating elements to form theretroreflection structure. Alternatively, the conical reflectors may beformed through laser modification of a single glass rod, using lasermaching methods used for forming precision conical surfaces within theglass rod. Other retroreflective cone structures may also beimplemented, and outcoupler and high-reflectance mirrors may be separateelements. Alternatively, the double-VHF-BR-based cavity of FIG. 32 couldbe used in a stacked array.

The substrates (501) and cooling plates (531) are preferably transparentto pump radiation when optical face-pumping is desired in a stackedconfiguration.

A waveguide structure (540) may be integrated into the disclosed planarVHF-BR-based cavity, in FIG. 34. In embodiments providing a SOIsubstrate, regions of the silicon cap layer are removed so that awaveguide structure (540) may be positioned adjacent to the VHF-BR-basedcavity. The waveguide core (542) and waveguide cladding (541) arepreferably positioned to allow evanescent coupling between the waveguidecore and the gain volume (505) of a VHF-BR-based cavity.

In a further embodiment, a multitude of light emitting diodes,preferably laser diodes, are symmetrically positioned over thedielectric layer, in FIG. 35, so that a radially symmetric pattern ofpump sources is produced. In this particular embodiment, pump sources(550) are located over the gain volume (505) and inside the innerdiameter of the VHF-BR region (504). Accordingly, a separate gain regionmay be attributed to each portion of the gain volume immediatelyunderlying each pump source, so that a multitude of gain regions result.

Such a radial pattern of pump sources may be produced by a variety ofmeans, including that of attaching individual pump modules to thedielectric layer. The symmetric pattern of pump sources provideparticular advantages when used in conjunction with the disclosedVHF-BR-based resonator, in that this multitude of pump sources may beoperated so that an intensity of pump radiation produced by each pumpsource is varied with time, so that pump radiation is provided aroundthe gain volume of the VHF-BR-based resonator in a sequential manner.

Since the various traveling and standing modes that may be supported bya particular resonator of the invention will each have a differentcharacteristic frequency, ω_(c), with which photons travel fullrevolutions around the cavity, one of these modes may be preferentiallyamplified by the pump sources by oscillating the multitude of pumpsources in concert with a particular desired mode. For example, inconjunction with the embodiments of FIG. 3, it may be seen that aplurality of modes may be supported within the solid cone of reflectionΔθ_(r), within which modes are reflected with adequate reflectivity toallow amplification. Aside from the magnitude of Δθ_(r), the number ofmodes allowed by this solid cone of reflection will also depend on suchother variables as the radius, r, of the cavity, and the modewavelength, λ_(c). Thus, given the wide range of possible values forthese variables, the frequencies, ω_(c), attributable to walking orstationary modes may have values ranging over many orders of magnitude,ranging from several hertz to many GHz.

In FIG. 35, the pump sources are positioned in opposing pairs about thecavity gain region, so that a ring-shaped array comprising a multitudeof pump sources is embodied. The pairs of pump sources are sequentiallylabelled as ‘A’, ‘B’, ‘C’, and so on, up to ‘R’. In the presentembodiment, these pump pairs are energized to emit optical power in anaccordingly sequential order, so that the gain volume is pumped in acircularly oscillating manner. This may be accomplished by providing asine-wave power to each of the pump pairs, wherein the phase of thesine-wave is shifted incrementally by an equal phase angle at eachsuccessive pump pair, relative to the last.

For example, if light traveling within a desired mode returns upon itspath in 1×10⁻⁷ seconds, so that ω_(c)=1×10⁷ Hz, then such a mode may bepreferentially pumped by the pump sources when the pump pairs ‘A’through ‘T’ are cycled at an equivalent frequency of 1×10⁷ sec⁻¹. Suchresonant pumping may be implemented in a wide variety of combinationsthat are less localized. For example, cycling at the same frequency maybe performed synchronously to each successive set of three pump pairs,[ABC], [DEF], [GHI], [JKL], [MNO], and [PQR], wherein each set is cycled(e.g., ABCABCABCA . . . ) with the desired frequency, ω_(c), and so thatA, D, G, J, M, and P, are in phase. It is furthermore apparent, then,that the choice of order in cycling the pump sources—i.e., whether theorder is [ABCABCA . . . ] or [CBACBAC . . . ]—will effect as to whether,so resonantly pumped, propagating light in the cavity is traveling in aclockwise or counterclockwise rotation. Such resonant pumping schemesmay be utilized to create helical beam paths when utilized inconjunction with conically-coupled embodiments of this disclosure. Thus,such cyclical powering of the pump sources provide an additionalmode-selection means, in addition to the mode-selection means providedby the high finesse of the VHF-BR. Also, there may be occurences whereinlight propagation within the cavity is not within a stationary mode ofthe cavity, yet is still amplified by the gain material, so that theVHF-BR-based cavity of the present embodiment may essentially act as anoptical amplifier for an injected signal. In this embodiment, aninjected signal may thus be used to modulate gain of the normal cavitymodes.

Such cyclical pumping may be performed in accordance with variousalternative embodiments, such as wherein the circular array of pumpsources are cyclically pumped with more than one cycle frequency so asto pump more than one mode, produce a beat frequency, promote aparticular harmonic, etc. In one alternative embodiment

Various devices may be contemplated using the principles and structuresset forth herein. For instance, in FIG. 36, several waveguides might bepositioned to evanescently or directly communicate with the planarVHF-BR-based cavities of FIGS. 26-36.

INDUSTRIAL APPLICATIONS

The present invention is seen to have potential applications in severalareas. One such application would be in the treatment of optical fibersor optical fiber preforms, where the fiber or preform could be passedthrough the center of a laser cavity similar to that described in FIG.3. Another potential application could arise in the general field ofvapor deposition, where various vapors or gases might be ionized,heated, or otherwise altered by passing through the process volume ofFIG. 3.

Other uses are suited to embodiments of the invention wherein telescopicoptics are used to centrally couple optical energy from the cavitycenter. The latter embodiments would be useful in any applicationwherein a laser beam is required, such as in telecommunications.

One could extrapolate from the discussed VHF-BR to propose an ultra-highfinesse (UHF) Bragg reflector, having yet higher finesse than thosereflectors contemplated herein. However, such further refinements wouldnot substantially alter the novel operational principles set forthherein, and would therefore comprise, in this context, a subset ofVHF-BR's.

Also, layers of the VHF-BR need not be precisely quarterwave, since verysmall cavities may benefit from the implementation of Bessel-typedistributions of layer thickness. Furthermore, solid state disk and slabcavities of the present invention may be pumped in any of theappropriate configurations discussed in the prior art, including, butnot limited to edge-pumping and face-pumping.

The preceding description provides an laser cavity structure that may beoperated as a laser, optical amplifier, or other, optically resonating,device. Although the present invention has been described in detail withreference to the embodiments shown in the drawings, it is not intendedthat the invention be restricted to such embodiments. It will beapparent to one practiced in the art that various departures from theforegoing description and drawings may be made without departure fromthe scope or spirit of the invention.

1. A light amplification device providing a preferred optical mode,comprising: a.) an annular reflective structure having a central axis,the reflective structure forming an optical cavity sustaining thepreferred mode, the annular reflective structure comprising a Braggreflector; b.) a gain medium within the cavity, the gain medium disposedto provide gain in the preferred mode, the gain medium encircled by theBragg reflector; and, c.) a pump source disposed to pump the gainmedium, the pump source disposed so as to provide a modulated excitationof the gain medium at a modulation frequency, wherein propagation in thepreferred mode rotates about the cavity with a rotation frequency, therotation frequency related to the modulation frequency, so that themodulated excitation results in the gain medium providing gainpreferentially to the preferred mode.
 2. The light amplification deviceof claim 1, the frequency in a frequency range, the range betweenseveral hertz and several gigahertz.
 3. The light amplification deviceof claim 1, wherein a substantially conical reflector is used fordirecting optical energy out of the cavity.
 4. The light amplificationdevice of claim 1, wherein the device is used to irradiate an opticalfiber.
 5. The light amplification device of claim 1, wherein the deviceis used to irradiate a photo-absorbing medium that is passed through acentral process space in the cavity.
 6. The light amplification deviceof claim 1, wherein the device is used to irradiate a target for energyproduction.
 7. The light amplification device of claim 1, wherein thereflective structure is discontinuous.
 8. The light amplification deviceof claim 1, wherein the device is a multi-mode laser.
 9. The lightamplification device of claim 1, wherein the device comprises ahigh-brightness source.
 10. The light amplification device of claim 1,wherein the preferred mode is non-stationary.
 11. The lightamplification device of claim 1, wherein the means for pumping isselected from the group consisting of gas discharge pumping,semiconductor pumping, and optical pumping.
 12. The light amplificationdevice of claim 1, wherein the device is used for opticalcommunications.
 13. light amplification device of claim 1, whereinenergy is coupled out of the cavity evanescently.
 14. The lightamplification device of claim 1, wherein the reflective structure isdisposed on a silicon substrate.
 15. The light amplification device ofclaim 1, wherein the gain medium is a solid.
 16. The light amplificationdevice of claim 1, wherein the gain medium is a gas.
 17. A lightamplification device providing a preferred optical mode, comprising: a.)an annular reflective structure having a central axis, the reflectivestructure forming an optical cavity sustaining the preferred mode, theannular reflective structure comprising a Bragg reflector; b.) a gainmedium within the cavity, the gain medium disposed to provide gain inthe preferred mode, the gain medium encircled by the Bragg reflector;c.) a first pump source disposed to pump the gain medium, the pumpsource disposed so as to provide a modulated excitation of the gainmedium, the modulated excitation having a first modulation frequency;and, d.) a second pump source disposed to pump the gain medium, thesecond pump source disposed so as to provide a modulated excitation ofthe gain medium at a second modulation frequency, the second frequencyhaving a phase difference relative to the first modulation frequency, sothat that the gain medium provides gain preferentially to the preferredmode.
 18. The light amplification device of claim 17, wherein the gainmedium is an array of separately pumped gain volumes.
 19. The lightamplification device of claim 17, wherein there is a harmonicrelationship between the rotation frequency and the modulationfrequency.
 20. The light amplification device of claim 17, wherein apump excites the gain medium with more than one modulation frequency.21. The light amplification device of claim 1, wherein a second Braggreflector is disposed within the cavity concentric to the annularreflective structure, such that an annular gain region is formed betweenthe annular reflective structure and the second Bragg reflector.